Conventionally, for patterning a solder resist for a printed wiring board, a contact exposure method in which a photomask is in contact with a solder resist is mainly used. However, with an increasing density of a printed wiring board, from the viewpoint of obtaining excellent alignment accuracy, a (direct) exposure or a divided projection exposure which does not use a photomask is used in recent years.
The direct exposure carries out a light exposure by direct scanning of a laser beam or the like. The divided projection exposure repeatedly carries out a light exposure using a projection type exposure device while carrying out alignment in a small exposure area. Since a solder resist layer on a patterned wiring board is subjected to scanning or repetitive exposure, a conventional solder resist having an appropriate exposure amount of 200 mJ/cm2 or more has a problem in that it takes a long period of time to perform exposure.
For this reason, a solder resist composition is suggested which can exhibit high photopolymerization performance. For example, disclosed in Patent Literature 1 is a photoresist ink obtained by adding an acrylic acid and further adding an acid anhydride to an epoxy group-containing polymer like a copolymer between glycidyl methacrylate and methyl methacrylate so as to produce a carboxyl group, and by consequently reacting the carboxyl group with 4-hydroxybutyl glycidyl acrylate. However, as the side chain molecule is very long compared to the main chain and also it is branched, there is a problem in that touch dryness of fingers of a dried coating film is extremely poor.
Further, disclosed in Patent Literatures 2 and 3 is a solder resist composition in which an oxime ester initiator is used as a photopolymerization initiator. With such solder resist composition, a good touch dryness of fingers can be obtained. However, according to the exposure method like the direct exposure, the sensitivity of a composition has a huge influence on the productivity, and therefore ultra-high sensitivity is required more than ever. There is also a problem that components of a photopolymerization initiator are volatilized as an out gas to cause contamination.
For example, according to the direct exposure or the divided projection exposure in which a photomask is not brought into contact with a solder resist layer, there is a possibility that the components of a photopolymerization initiator volatilize as an out gas during an exposure, thereby contaminating optical elements within an exposure device. Further, to obtain good solder heat resistance, post-curing like thermal curing or UV exposure is generally carried out after developing the solder resist layer, and mounting is carried out by re-flow during a post process. At that time, the components of a photopolymerization initiator volatilize as an out gas, cool, and solidify to serve as a cause for contamination of a work environment.
Under the circumstances, various methods to suppress an out gas are studied (for example, see Patent Literature 4 and the like). However, according to high densification and high performance of a printed board, further suppression of contamination is required and it is required more and more to suppress the generation of an out gas.